Conventionally, the pointing device is used to locate a specific point on a computer display, or input tracks for characters or graphics to be displayed on the display. The pointing device comes in various forms as in a mouse, a pen, and a trackball, for example.
Recently, there has been proposed an interesting pen-type pointing device employing ultrasonic wave.
As proposed, a coordinates input system includes, for example, a pen device 110 equipped with an ultrasonic transmitter T on a tip 110a, and a receiver unit 150 including two ultrasonic wave receivers R1 and R2 and connected to a computer 200, as illustrated in FIG. 2. With the coordinates input system, the position information of the pen device 110 is input to the computer 200.
In this system, the velocity of the ultrasonic wave transmitted from the ultrasonic wave transmitter T, and the distance P between the ultrasonic wave receivers R1 and R2 are fixed.
The receiver unit 150 determines a propagation time t1 of the ultrasonic wave transmitted from the ultrasonic wave transmitter T and received by the ultrasonic wave receiver R1, and a propagation time t2 of the ultrasonic wave transmitted from the ultrasonic wave transmitter T and received by the ultrasonic wave receiver R2. By multiplying the velocity of the ultrasonic wave with the propagation time t1 and t2, the distance L1 between the ultrasonic wave transmitter T and the ultrasonic wave receiver R1, and the distance L2 between the ultrasonic wave transmitter T and the ultrasonic wave receiver R2 can be obtained.
FIG. 3 represents a positional relationship between the ultrasonic wave transmitter T and the ultrasonic wave receivers R1 and R2, in which the ultrasonic wave receiver R1 is the origin of the coordinates system. In the coordinates system shown in FIG. 3, if the coordinates of the ultrasonic wave transmitter T is (x, y) in a triangle with apices R1, R2, and T, then triangulation gives:x2+y2=L12  (1)(x−p)2+y2=L22  (2).
By transforming Equations (1) and (2), the following Equations (3) and (4) can be obtained.x=(P2+L12−L22)/2P  (3)y=(L12−x2)1/2  (4).The coordinates (x, y) of the ultrasonic wave transmitter T can be obtained by substituting the distance L1, L2, and the pre-defined distance P in Equations (3) and (4), where L1 and L2 are distances respectively obtained from the propagation times t1 and t2 calculated in the receiver unit 150. Since the ultrasonic wave transmitter T is installed in the pen device 110, the coordinates (x, y) can be regarded as the coordinates of the pen device 110. Thus, the coordinates (position information) of the pen device 110 can be obtained by finding the coordinates of the ultrasonic wave transmitter T.
In a coordinates input system (coordinates input device) disclosed in Japanese Laid-Open Patent Publication No. 132436/2002 (Tokukai 2002-132436; published on May 10, 2004), the coordinates of pen device 110 is determined by the following procedure.
First, the propagation time t1 of the ultrasonic wave transmitted from the ultrasonic wave transmitter T and received by the ultrasonic wave receiver R1 is calculated, and the propagation time t1 is multiplied by the velocity of the ultrasonic wave so as to obtain distance L1 between the ultrasonic wave transmitter T and the ultrasonic wave receiver R1. Then, a phase difference between the ultrasonic wave received by the ultrasonic wave receiver R1 and the ultrasonic wave received by the ultrasonic wave receiver R2 is calculated, and the distance between L1 and L2 is determined from the phase difference. The distance L2 between the ultrasonic wave transmitter T and the ultrasonic wave receiver R2 is then obtained from the distance L1 and the difference between the distance L1 and distance L2. Then, the coordinates (x, y) of the ultrasonic wave transmitter T are obtained by substituting the distance L1, L2, and P in Equations (3) and (4), thereby obtaining the coordinates of the pen device 110 equipped with the ultrasonic wave transmitter T.
Further, in the coordinates input system of the foregoing publication, the propagation time t1 of the ultrasonic wave transmitted from the ultrasonic wave transmitter T and received by the ultrasonic wave receiver R1 is obtained as follows.
First, in the pen device 110, the ultrasonic wave transmitter T transmits an ultrasonic wave, and an infrared emitting element transmits an infrared synchronization signal indicative of a transmission timing of the ultrasonic wave.
The ultrasonic wave receiver R1 receives the ultrasonic wave, and determines an envelope of the received ultrasonic wave. Then, the transmission timing of the ultrasonic wave is detected based on the infrared synchronization signal, so as to obtain a propagation time of the ultrasonic wave, which is the time period from the transmission timing of the ultrasonic wave to the timing at which the envelope exceeds a predetermined threshold (see paragraph [0024] and FIG. 5 of Tokukai 2002-132436).
However, the propagation time obtained by the foregoing process has error variations according to the distance between the ultrasonic wave receiver R1 and the ultrasonic wave transmitter T. This is described below in detail.
The characteristics of the received ultrasonic wave are such that the overall height of the amplitude fluctuates depending on the distance between the transmission source and the receiver. Thus, with the predetermined threshold held at a constant level, the time period from the reception of the ultrasonic wave to the point where the envelope exceeds the predetermined threshold varies depending on the distance between the ultrasonic wave transmitter T and the ultrasonic wave receiver R1, with the result that the propagation time of the ultrasonic wave is obtained with error variations.
More specifically, when the distance between the ultrasonic wave transmitter T and the ultrasonic wave receiver R1 changes, the position of the amplitude that exceeds the envelope may be changed in the received ultrasonic wave. This changes the time period from the reception of the ultrasonic wave to the point where the envelope crosses the predetermined threshold, with the result that the resulting propagation time varies depending on the distance. (For example, with an ultrasonic wave of a certain propagation distance, the envelope exceeds a predetermined threshold at the rise of the first amplitude. However, if the propagation distance is changed, the envelope may exceed the predetermined threshold at the rise of the third amplitude.)
In the manner described above, the propagation time of the detected ultrasonic wave is obtained with error variations depending on the position of the pen device 110 relative to the ultrasonic wave receiver R1. There according will be error variations in the coordinates of the pointing device calculated based on the propagation time.